1. Field of the Invention
This invention relates to a method for measuring viscosity of liquid, and a method and an apparatus for measuring visco-elasticity of liquid, in which viscosity and visco-elasticity of liquid to be measured can be obtained correctly using a vibration-type liquid detecting apparatus.
2. Related Art
A vibration-type liquid detecting apparatus U as represented by Japanese Patent Publication No. H05-20692 employs, as shown in FIG. 2, a method for measuring viscosity of liquid, in which vibration of a piezoelectric vibrator 1 composed of a piezoelectric element is transmitted to a liquid detecting piece 3 through a vibration transmission shaft 2 so as to be resonated, the liquid detecting piece 3 is immersed and vibrated in liquid 4 to be measured so that it can detect viscous resistance of the liquid, and then amplitude of the vibration corresponding to the viscous resistance is detected by an angular acceleration sensor 5 composed of a piezoelectric element disposed at an end of the vibration transmission shaft 2.
Specifically, a curve of a static viscosity (viscous property)-to-amplitude of vibration in a plurality of viscosity standard liquids (Newtonian liquid defined in JIS) is obtained beforehand. On the other hand, an actual amplitude of vibration of liquid to be measured is obtained. Then, a viscosity corresponding to the amplitude on the curve of the static viscosity (viscous property)-to-amplitude is regarded as the viscosity of the liquid to be measured. In other words, it employs a method in which the amplitude is regarded as a variable determining the viscosity.
As mentioned above, a static viscosity is obtained from vibration of a liquid detecting piece in liquid and the viscosity thus obtained is used as a viscosity of the liquid. This measuring method can be the correct method, only on the assumption that the liquid to be measured, such as water, gasoline, oil and the like which is generally handled as Newtonian liquid is a liquid truly equivalent to the Newtonian liquid.
On the other hand, a polymeric solution such as a photosensitive agent and a liquid enriched with visco-elasticity such as a coating material for paper are also regarded as Newtonian liquid and the measuring method based on the Newtonian liquid is applied thereto. However, since those liquids are enriched with visco-elasticity which is not expected in the Newtonian liquid, it is hard to say that the viscosity obtained according to the aforementioned measuring method reflects an actual viscosity correctly.
As a result of repeated measurement of viscosity of the non-Newtonian liquid and the Newtonian liquid utilizing a liquid detecting piece having various resonant frequencies, the inventor of the present invention has found out that the dynamic viscosity detected by vibration of the liquid detecting piece is an important element as an element for measuring viscosity and visco-elasticity of liquid, the dynamic viscosity in those liquids is not necessarily equivalent to the static viscosity, a difference between the dynamic viscosity and the static viscosity becomes more significant as the frequency of vibration is increased (dynamic viscosity is highly dependent on frequency), and therefore, the dynamic viscosity reflects viscosity and visco-elasticity through frequency.
Based on the above finding, there is provided a method for measuring viscosity or visco-elasticity in which a liquid detecting piece is vibrated by a vibrator, vibration in the liquid is detected, and viscosity or elasticity of the liquid is measured from vibration thus obtained, the method comprising the steps of detecting frequency and amplitude which are variable attributable to liquid-specific visco-elasticity from the vibration, obtaining impedance of the liquid from the frequency and the amplitude, and obtaining a dynamic viscosity or a dynamic elasticity from a real number part and an imaginary number part of the impedance.
The method may comprise the steps of detecting frequency and amplitude which are variable attributable to liquid-specific visco-elasticity from the vibration, obtaining impedance of the liquid from the frequency and the amplitude, and obtaining a dynamic viscosity, a dynamic elasticity and a static elasticity of the liquid from a real number part and an imaginary number part of the impedance.
By the above-mentioned method, there can correctly be obtained a viscosity and a visco-elasticity of various kinds of liquid to be measured ranging from a liquid having no visco-elasticity and to a liquid enriched with visco-elasticity in a case in which a vibration-type liquid detecting apparatus having such a reciprocal vibration mode as a circular direction vibration.
From another aspect of the present invention, there is also provided an apparatus for measuring visco-elasticity of liquid comprising a liquid detecting piece resonated in liquid serving a vibrator as a drive source, a circuit for detecting frequency of vibration of the liquid detecting piece in the liquid, a circuit for detecting amplitude of the vibration, a circuit for computing impedance of the vibrator from the frequency and the amplitude of the vibration, and a circuit for computing a dynamic viscosity, a dynamic elasticity and a static viscosity of the liquid from a real number and an imaginary number of the impedance computed by the impedance computing circuit.
Particularly, according to the present invention, there are provided a measuring method and an apparatus, in which visco-elastic properties (viscosity and elasticity) of liquid, which directly affects the physical properties of such polymeric solutions as silicon oil and synthetic resin liquid as a raw material of synthetic resin products, can easily be measured accurately and in real time.